%0 Journal Article %T Ortho-Para Selection Rules in the Gas-phase Chemistry of Interstellar Ammonia %+ Institut de Planétologie et d'Astrophysique de Grenoble (IPAG ) %+ Institut d'astrophysique spatiale (IAS) %A Faure, A. %A Hily-Blant, P. %A Le Gal, R. %A Rist, C. %A Pineau Des Forêts, G. %< avec comité de lecture %J The Astrophysical Journal Letters %V 770 %8 2013 %D 2013 %Z 2013ApJ...770L...2F %R 10.1088/2041-8205/770/1/L2 %K astrochemistry %K ISM: molecules %K molecular processes %Z Sciences of the Universe [physics]Journal articles %X The ortho-para chemistry of ammonia in the cold interstellar medium is investigated using a gas-phase chemical network. Branching ratios for the primary reaction chain involved in the formation and destruction of ortho- and para-NH3 were derived using angular momentum rules based on the conservation of the nuclear spin. We show that the "anomalous" ortho-to-para ratio of ammonia (~0.7) observed in various interstellar regions is in fact consistent with nuclear spin selection rules in a para-enriched H2 gas. This ratio is found to be independent of temperature in the range 5-30 K. We also predict an ortho-to-para ratio of ~2.3 for NH2. We conclude that a low ortho-to-para ratio of H2 naturally drives the ortho-to-para ratios of nitrogen hydrides below the statistical values. %G English %L insu-03617070 %U https://insu.hal.science/insu-03617070 %~ INSU %~ UNIV-SAVOIE %~ UGA %~ CNRS %~ UNIV-GRENOBLE1 %~ UNIV-PSUD %~ INPG %~ IRSTEA %~ CNES %~ OSUG %~ IPAG %~ UNIV-PARIS-SACLAY %~ AGREENIUM %~ UNIV-PSUD-SACLAY %~ INRAE %~ USMB-COMUE %~ GS-PHYSIQUE %~ IAS